In integrated circuits including complementary metal oxide semiconductor (CMOS) integrated circuits, it is often desirable to be able to permanently store information, or to form permanent connections on the integrated circuit after it is manufactured. Fuses or other devices forming fusible links are frequently used for this purpose. Fuses can be used to program redundant elements to replace identical defective elements, for example. Fuses can also be used to store die identification or other information, or for other applications.
Fuse devices are selectively programmed to provide the capabilities described above. Where one or more fuses is used for information storage purposes, a sensing circuit is typically used to determine whether an associated fuse has been programmed or not, i.e. a sensing circuit is used to determine the value "stored". Sensing circuits operate to distinguish between programmed and unprogrammed (or burned and un-burned) fuses, usually by detecting a change in the resistance of the fuse device from a low to a high value.
Advances in process technology have introduced some new issues with respect to sensing the states of fuses. The operating and junction breakdown voltages of the latest semiconductor manufacturing processes and those in development continue to decrease. Fuses requiring high programming currents and voltages, or thick gate oxides for reliable operation, are therefore not viable for use in many of the newest integrated circuit devices. Thus, new fuses are being developed to be compatible with the constraints of new and upcoming processes.
Another issue arises because of the smaller geometries provided by new and upcoming process technologies. Smaller geometries include smaller line widths and device sizes. At reduced geometries, the resistance of the fuse device is more difficult to control. In some cases, the difference in resistance between a burned fuse and an unburned fuse may be so small that the difference is difficult to detect with prior fuse sensing circuits.
Fuses having a lower programming current may have a wide range of programmed resistance values as compared to their unprogrammed resistance values. Further, the change in resistance of such fuses from the unprogrammed to the programmed state may be as small as 1.5.times.for one type of fuse. A low post-burn resistance creates difficulties in sensing the state of a fuse, particularly for a low voltage CMOS process, for example. Merely increasing sensing current to increase sensing sensitivity is not a viable approach. If the current through an unprogrammed fuse is not low enough during sensing, the unprogrammed fuse may be erroneously programmed.
Prior fuses and fuse sensing circuits present another disadvantage where lower voltage processes are used. As processes move to lower supply voltages, the voltage available to program fuses also inherently decreases. As the fuse programming voltage is lowered, the number of "marginally burned" fuses increases. Fuses are considered to be marginally burned when, after programming, the resistance of the fuse remains low enough that there is an unacceptable risk that the fuse might be identified as being unprogrammed when its state is sensed. Therefore, marginally burned fuses may compromise the functionality or quality of the circuit that uses the fuse. This is particularly true where the state of a single fuse determines the state of a fuse-based storage cell. Additional or redundant fuses have previously been provided for this type of cell, but each redundant fuse takes up valuable space on the integrated circuit die. Further, redundancy may still not provide adequate yield for some processes.
Thus, fuses having low programming currents, and/or fuses that exhibit a small change in resistance between an unburned state and a burned state, can present difficulties in terms of providing a safe and reliable fuse-based storage cell.
One type of prior fuse-based storage cell, for example, includes a pair of fuses forming one side of a four resistor bridge circuit. The opposite side is formed by two reference resistors. One fuse device is programmed if one circuit state is desired (a logical "1", for example), and the opposite fuse device is programmed if the opposite state is desired. For low voltage CMOS processes, however, when sensing the state of this type of circuit, the programmed state of one of the fuses can cause the current through the other fuse device to become high enough to program or partially program the unprogrammed device. In this manner, sensing the states of fuses can cause the circuit including the fuses to become unreliable.
Another type of sensing circuit uses a predetermined, non-variable resistance to establish a reference voltage in one branch of a sensing circuit. The state of a fuse in another branch of the sensing circuit is then identified by determining whether a voltage in the second circuit branch is higher or lower than the reference voltage. Examples of such circuits are described in U.S. Pat. No. 4,730,129 to Kunitoki et al. and U.S. Pat. No. 5,384,746 to Giolma. A disadvantage of this arrangement is that, in low voltage CMOS processes, for example, the distribution of post-burn fuse resistances can be very wide. The sensitivity of this type of circuit decreases with a lower supply voltage and may result in a lower yield or even lower reliability.
Other types of prior fuse sensing circuits have poor controllability and thus, rely on tight manufacturing controls to ensure a predictable output state during sensing. In this case, manufacturing variations can cause the output produced during fuse sensing to be indeterminate, and therefore, unreliable. For this reason, while maintaining a low sensing current and voltage, it is also important to ensure reliable measurement by compensating for variations that can cause anomalous sensing at low voltage levels.
This can be an issue particularly in the case of CMOS integrated circuit devices, where it is difficult to directly and reliably sense signals below about 100 mV due to random and systematic variations in device threshold voltages (Vt) and effective channel lengths (Le). Further, prior fuse sensing circuits having poor controllability also have the potential to create unsafe currents for a fuse having a low programming current.